Chemical composition of iron and steel slag

The primary components of iron and steel slag are limestone (CaO) and silica (SiO2). Other components of blast furnace slag include alumina (Alsub>2O3) and magnesium oxide (MgO), as well as a small amount of sulfur (S), while steelmaking slag contains iron oxide (FeO) and magnesium oxide (MgO). In the case of steelmaking slag, the slag contains metal elements (such as iron) in oxide form, however because the refining time is short and the amount of limestone contained is large, a portion of the limestone auxiliary material may remain undissolved as free CaO.
These components exist in the natural world in places such as the Earth’s crust, natural rock, and minerals, and the chemical composition is similar to that of ordinary Portland cement. The shape and physical characteristics of iron and steel slag are similar to ordinary crushed stone and sand, however due to differences such as the chemical components and cooling processes, it is possible to provide different types of slag with a wide variety of unique properties. For example, there are some types of slag that harden when alkali stimulation occurs. Many applications utilizing the physical and chemical characteristics of slag have been developed and are being put to use in a broad range of fields.

Examples of iron and steel slag compositions

(Unit: %)

Type

Blast furnace slag

Converter slag

Electric arc furnace slag

Andesite (for reference)

Ordinary cement

Component

Oxidizing slag

Reducing slag

CaO

41.7

45.8

22.8

55.1

5.8

64.2

SiO2

33.8

11.0

12.1

18.8

59.6

22.0

T-Fe

0.4

17.4

29.5

0.3

3.1

3.0

MgO

7.4

6.5

4.8

7.3

2.8

1.5

Al2O3

13.4

1.9

6.8

16.5

17.3

5.5

S

0.8

0.06

0.2

0.4

-

2.0

P2O5

<0.1

1.7

0.3

0.1

-

-

MnO

0.3

5.3

7.9

1.0

0.2

-

Dissolution characteristics

pH

Due to the effects of the limestone it contains, the pH of iron and steel slag increases to 10 – 12 when it reacts with water, indicating alkalinity that is the same or lower than that of recycled concrete base course material and cement-stabilized soil.
Because the soil in Japan is generally acidic, the alkali components that dissolve from iron and steel slag products are absorbed and neutralized by the soil. In instances when water that contacts the slag does not pass through the soil and may flow directly to the outside, then in the same way as with recycled concrete base course material and cement-stabilized soil, it is necessary to take steps to reduce alkalinity. This can include creating an embankment of soil with high alkali-absorption capacity, or neutralizing the water with carbon dioxide gas before draining. When using an iron and steel slag product, be sure to consult in advance with the manufacturer and seller of the product regarding necessary water drainage measures during and after construction.

Heavy metals

When iron and steel slag products are used on land, environmental safety is verified based on the Environmental Quality Standards for Soil Contamination. When the products are used in a marine environment or landfill, safety is verified based on the benthic soil standards in the Law on the Prevention of Marine Pollution and Maritime Disaster.
Although the quality of iron and steel slag products is already prescribed by JIS as materials for use in civil engineering works, because there was previously no quality standard related to environmental safety, no specific environmental considerations had been prescribed for these products. Subsequently Test Methods for Chemicals in Slags (JIS K0058-1) was established in 2005, and at present environmental considerations are being added to or revised in JIS for iron and steel slag used in roads and other iron and steel slag products.